(1) Field of the invention
The present invention relates to a sliding material for forming a sliding member and to a sliding bearing composed of the sliding material, and relates to a sliding material for forming a sliding member that is used in a clean atmosphere and a low-pressure atmosphere such as under vacuum, and the like, and a sliding bearing which can be suitably used for a mating member made of a soft material or a sliding bearing demanded to have a high rotational accuracy.
(2) Description of Related Art
Including information disclosed under 37 CFR1.97 and 1.98
The sliding member obtained by molding the sliding material such as a resin composition having lubricating property is severely demanded to have higher function year by year, and is strongly demanded to have a low friction and low wear in an initial state and maintain an initial sliding property thereof for a long time. When a resinous material containing the lubricating oil is used as the above-described sliding material, improvement of the sliding property thereof has been aimed by adjusting the affinity of lubricating oil for resin serving as a base material, mixing a filler serving as a conductive path of the lubricating oil with the resin, and adjusting a mixing condition. In recent years, there is disclosed a sliding material whose sliding property has been improved by using the resinous material (patent document 1) mixed with the porous silica impregnated with the lubricating oil.
In a sliding material which is used in a closed clean atmosphere such as in semiconductor-manufacturing equipment and particularly in a sliding material which is used in a low-pressure clean atmosphere such as under vacuum, vapor generated from a lubricant and scattered fine particles may adversely affect the performance of precision parts. Thus the sliding material is demanded to have a very low out particle performance in addition to the above-described demanded properties. As a filler for use in the sliding material, a liquid lubricant having a low vapor pressure; a solid lubricant such as polytetrafluoroethylene, molybdenum disulfide, tungsten disulfide, and the like; and a soft metal such as gold, silver, and the like are used.
To prevent the lubricating oil from being decomposed when the conventional sliding material containing the lubricating oil is molded into the sliding member, it is necessary to select a lubricating oil heat-resistant to a molding temperature of resin. Thus “super engineering plastics” having a high molding temperature cannot be used as the resin.
When the sliding material is used for a member to be used in a vacuum condition, the conventional sliding material has a problem that even when the vapor pressure of the lubricating oil used therefor is low to such an extent that it is capable of withstanding a vacuum condition, the sliding material cannot be used when the decomposition temperature of the lubricating oil is low.
In improving the mechanical strength of the resinous material by mixing a reinforcing material made of fiber or the like with resin, the wetting property of the fiber with the resin is important. It is difficult to sufficiently improve the strength of the resinous material owing to the influence of the lubricating oil mixed with the resin. To secure a predetermined supply amount of the resinous material without slipping the resin on a screw in an injection molding time, the amount of the lubricating oil which can be mixed with the resin is up to a maximum of about 10 vol %. In dependence on a use condition, the amount of the lubricating oil may be short.
In the resinous material (patent document 1) mixed with the porous silica impregnated with the lubricating oil, the mixing amount of the lubricating oil is allowed to be larger than that of the conventional resinous material. Even in this resinous material, the mixing amount of the lubricating oil is up to a maximum of 30 vol %. In environment where a member composed of the resinous material is used in a severe condition, there is a possibility that the lubricating oil is short. Further, the problem of selecting the resin and the lubricating oil remains unsolved. Thus it is difficult to use the resinous material in environment where the resinous material is required to be heat-resistant.
It cannot be said that the sliding material for a solid body-lubricating method used in the above-described clean atmosphere and low-pressure atmosphere such as in the semiconductor-manufacturing equipment has a sufficient sliding performance in a severe use condition, for example, a high load and a high speed. For example, because the polytetrafluoroethylene melts and wears owing to heat generated thereby in a high speed condition, the polytetrafluoroethylene has a low wear resistance. The molybdenum disulfide is used mainly as a coating material. In a high load condition, a thin coating film is liable to peel off.
As an application example of the oil-containing sliding material, sliding bearings such as an oil-containing bearing composed of a sintered metal and an oil-containing bearing composed of a resin are known. The oil-containing bearing composed of the sintered metal has an advantage that it does not wrap around a mating shaft and can be processed with a high accuracy, but has a disadvantage that it wears the mating shaft made of a soft metal. The oil-containing bearing composed of the resin has an advantage that it has self-lubricating property by dispersing the lubricating oil in a resin molding and does not attack a mating member made of a soft material, but is composed of a resinous material having a larger coefficient of linear thermal expansion and a larger coefficient of water absorption than a metal material. Thus when a service temperature has a wide region, the bearing wraps around the shaft owing to a contraction of the resin at a low temperature. Further at a high temperature, the expansion of the volume thereof is restrained by an outer-diameter side housing and hence shifts toward an inner-diameter side of the bearing. Thereby the inner diameter thereof becomes small, and the sliding bearing wraps around the shaft.
Therefore the development of a sliding bearing has been made by utilizing the advantages of both bearings and covering up the disadvantages thereof. For example, as an application to a field demanded to have a high rotational accuracy, a sliding bearing having a plurality of layers composed of the metal material layer and the coating layer of resin, having a thickness of about 20 μm, which is applied to the surface of the metal material layer is disclosed. Also disclosed is a sliding bearing having a high dimensional accuracy and not wearing the mating member made of a soft material. The sliding bearing has the porous resin layer formed as the sliding surface which slides on a shaft, and the sintered metal layer, disposed at the outer-diameter side of the sliding bearing, which serves as the lubricating oil supply layer (patent document 2).
In recent years, the sliding bearing for use in office appliances are severely demanded year by year to have a higher extent in the rotational accuracy and durability thereof. Thus even in the sliding bearing having a plurality of layers, when the solid lubricant becomes ineffective, the coating film peels off the surface of the metal material, and thus the metal material disposed below the coating film is exposed. Thus the sliding bearing is inferior in its durability. Even in the case where the sliding surface consists of the porous resin layer, the sliding bearing is incapable of obtaining the sliding property satisfying the above-described demanded performance in dependence on an interconnected hole porosity of the porous resin layer.
Patent document 1: Japanese Patent Application Laid-Open No.2002-129183
Patent document 2: Japanese Patent Application Laid-Open No.2002-364647
A sliding material of the present invention includes a resinous porous article having an interconnected hole porosity of not less than 30% and a lubricating oil which impregnates the resinous porous article. The resinous porous article has an interconnected hole formed by molding a resin mixed with a pore-forming substance into said resin, and extracting said pore-forming substance from said molded article with a solvent which dissolves said pore-forming substance and does not dissolve said resin.
It is an object of the present invention to allow a sliding material containing a lubricating oil to have a high mechanical strength and contain the lubricating oil at a high ratio and combine the sliding material and the lubricating oil with each other at a desired ratio in dependence on a use and a specification.
It is another object of the present invention to provide a sliding material in which a gas is not generated from the lubricating oil, even when the sliding material is used under vacuum and which is low out particle.
It is still another object of the present invention to provide a sliding bearing composed of the above-described sliding material and particularly a sliding bearing which can be suitably used for a mating member made of a soft material or a sliding bearing demanded to have a high rotational accuracy.